Necessary to find generated files in the out-of-tree build directory.
E.g. `tests/configurehelp.pm`, for tests 1119 and 1167.
Before this patch macOS autotools builds were failing these two tests
due to falling back to the default preprocessor (`cpp`) instead of
the actual one configured. Then `cpp` failing to compile Apple SDK
headers referenced by curl headers.
Cherry-picked from #14097
Closes #14124
# paths
our $pwd = getcwd(); # current working directory
our $srcdir = $ENV{'srcdir'} || '.'; # root of the test source code
-our $perl="perl -I$srcdir"; # invoke perl like this
+our $perl="perl -I. -I$srcdir"; # invoke perl like this
our $LOGDIR="log"; # root of the log directory; this will be different for
# each runner in multiprocess mode
our $LIBDIR="./libtest";